Field of the Invention
This invention relates in general to the field of optical probes and, in particular, to a novel non-contact interferometric optical probe particularly suitable for coordinate measuring machines (CMMs).
Description of the Prior Art
Coordinate measuring machines are used for measuring the physical geometrical characteristics of an object, typically for quality control purposes in manufacturing and assembly processes. The typical machine takes measurements along three axes (X, Y, Z), orthogonal to each other in a conventional three-dimensional coordinate system. The first horizontal axis (typically the Y axis) is defined by a bridge or gantry supported by two vertical legs coupled to a stationary support table. The second axis, X, is defined by the horizontal motion of the bridge along the support table in the direction normal the first axis (thereby defining an XY plane). The third, Z axis is defined by the vertical motion of a quill or spindle attached to the bridge. A probe is attached to the quill for single-point contact or optical measurements of an object based on a scale system that indicates the location of the probe along each axis. In operation the machine reads the input from the probe as it traces the part at various points and the X,Y,Z coordinates of these points are used to determine size and shape with micrometer precision.
The probe can be mechanical, where the position is established by touching the surface of the object, or non-contact, where the presence of the surface is sensed by observing changes in a particular physical quantity such as reflected light, magnetic field, etc. Mechanical probes require contact with the surface, which in many cases is not desirable, in particular in the measurement of precise optical components.
Many non-contact probes are optics-based. They typically provide good sensitivity but are limited by the amount of light reflected by the tested surface and require proper positioning relative to the surface to achieve optimal sensing conditions. In practice this means that a limited range of surface slopes can be reliably detected, which requires the repeated reorienting of the probe to place it substantially perpendicular to the tested surface, a task that makes the measurement systems less accurate and more complex. Furthermore, the ability to reorient the probe through rotation requires calibration of additional critical parameters in the operation of a coordinate measurement machine, with attendant uncertainties and potential errors.
It is therefore desirable to build a new type of probe capable of accepting a wide range of sensing angles at high accuracy and sensitivity. In this disclosure a new design of optical surface-sensing probe is described that has a large range of acceptance angles and hence can measure a wider range of surface slopes than, and overcome the limitations of, prior-art optical probes. The invention is based on the concepts and teachings disclosed in U.S. Pat. Nos. 8,422,026, 8,810,884 and 8,675,205, hereby all incorporated by reference, combined with a probe structure that enables practical embodiments.
Interference fringes localized relative to a reference surface are used to detect the test surface position. As used herein, “localized fringes” is intended to mean, in the case of low-coherence light sources, interference fringes formed in a limited space around the location where the optical path difference (OPD) between the test and reference beams is close to zero; i.e., where the delay between the reference and test beams is very small. In the case of spectrally-controlled or multiple-wavelength sources, “localized fringes” is intended to mean unambiguously identifiable fringe patterns formed at a predetermined distance from the reference surface. Throughout this disclosure, the word “localized” and related terms are used for convenience to describe the position of interferometric fringes in space in relation to the reference mirror of the interferometer, but it is understood that such fringes are only virtual fringes and that actual fringes are in fact formed on the instrument's detector only when the test surface is located at such “localized” position in space.